Graphics.Pgm is a pure Haskell library to read and write PGM images. It
properly supports both 8 bit and 16 bit pixels, and multiple PGMs per
file. The PGM is the lowest common denominator of useful image file
formats. It consists of a header of the form

P5 width height maxVal

followed by a single whitespace character, usually a newline, where
width, height, and maxVal are positive integers consisting of digits
only giving the number of columns, number of rows, and the highest grey
level in the image to follow.

If maxVal < 256, then the format uses 1 byte per pixel; otherwise it
uses 2. The routines in this library properly handle both, including
automatically determining which to write when writing an array to disk.

The header can also contain comments, starting with # on a new line, and
continuing to the end of the line. These are read out and returned as a
String with newlines kept intact (except for the last newline of the
last comment line, which is removed). Comments from anywhere between the
header fields are concatenated into the same document. pgmToArray
ignores comments; pgmToArrayWithComments reads them.

After the header, the pixel data is written in big-endian binary form,
most significant byte first for 16 bit pixels. The pixels are a single
row-major raster through the image.

To put multiple PGMs in a file, append them. This module allows you to
put white space between them, though this might choke other
implementations.

All arrays returned by this library from PGMs have pixel type Int, since
this is simply more useful for most purposes. If you want to write a PGM
back out, you must first coerce your pixel type to Word16! There are
too many possible ways of handling negative values, larger depths, or
other things beyond the comprehension of Word16 to handle with a simple
wrapper function. If you know you have positive values less than 2^16,
then you can coerce an array arr to Word16 with

amap (fromIntegral :: Int -> Word16) arr

The array's indices (of the form (row,column)) start at (0,0) and run to
(height-1,width-1).